Estrous Cycle and Stress: Influence of Progesterone on the Female Brain T.A

Home , Estrous cycle, The Female Brain (book)

ISSN 0100-879X Volume 45 (4) 291-375 April 2012

www.bjournal.com.br BIOMEDICAL SCIENCES

Braz J Med Biol Res, April 2012, Volume 45(4) 314-320 doi: 10.1590/S0100-879X2012007500044

Estrous cycle and stress: influence of progesterone on the female brain T.A. Lovick

The Brazilian Journal of Medical and Biological Research is partially financed by

Institutional Sponsors

Explore High - Performance MS Orbitrap Technology In Proteomics & Metabolomics Campus Ribeirão Preto Faculdade de Medicina de Ribeirão Preto

analiticaweb.com.br S C I E N T I F I C

All the contents of this journal, except where otherwise noted, is licensed under a Creative Commons Attribution License Brazilian Journal of Medical and Biological Research (2012) 45: 314-320 ISSN 0100-879X Review

Estrous cycle and stress: influence of progesterone on the female brain

T.A. Lovick

School of Clinical and Experimental Medicine, University of Birmingham, Birmingham, UK

Abstract

The female brain operates in a constantly changing chemical milieu caused by cyclical changes in gonadal hormones during the estrous cycle (menstrual cycle in women). Such hormones are highly lipophilic and pass readily from the plasma to the brain where they can influence neuronal function. It is becoming clear that the rapid reduction in peripheral circulating progesterone, which occurs during the late diestrous phase of the cycle, can trigger a withdrawal-like response, in which changes in GABAA receptor expression render hyper-responsive certain brain areas involved in processing responses to stressful stimuli. The periaqueductal gray matter (PAG) is recognised as an important region for integrating anxiety/defence responses. Withdrawal from progesterone, via actions of its neuroactive metabolite allopregnanolone, triggers up-regulation of extrasynaptic GABAA receptors on GABAergic neurons in the PAG. As a consequence, ongoing GABAergic tone on the output cells decreases, leading to an increase in functional excitability of the circuitry and enhanced responsiveness to stressful stimuli during the late diestrous phase. These changes during late diestrus could be prevented by short-term neurosteroid administration, timed to produce a more gradual fall in the peripheral concentration of allopregnanolone than the rapid decrease that occurs naturally, thus removing the trigger for the central withdrawal response.

Key words: Female; Estrous cycle; Stress; Anxiety; Progesterone; Neurosteroid replacement

Responsiveness to psychological stress in family relationships and other social activities. Interestingly, females PMS does not occur in anovulatory cycles (4,5), suggesting that cyclical changes in sex hormones, rather than absolute Adverse responsiveness to psychological stress is a levels may be the trigger for the syndrome. The late luteal major health hazard, which is more prevalent in women phase of the menstrual cycle is characterised by a rapid than in men (1). The female brain, unlike its male coun- decline in secretion of the sex hormones progesterone and terpart, operates in a changing chemical milieu caused by estrogen, suggesting that one or both of these hormones cyclical changes in production of female sex hormones in may be linked to the development of symptoms. This article the periphery during the menstrual cycle (estrous cycle in will focus on the recent evidence obtained in rodents that animals). Many of these compounds are neuroactive and indicates that the fall in progesterone secretion, which occurs their lipophilic nature enables them to pass readily through during the late diestrous phase in rats, leads to a change the blood brain barrier and produce significant influences in gamma-aminobutyric acid (GABAA) receptor expression on brain function through actions at both membrane-bound in the brain that is manifested behaviourally as enhanced and nuclear receptors (2). In large numbers of women, the responsiveness of the animal to mild anxiogenic stress. A late luteal phase of the menstrual cycle is characterised consideration of the effect of estrogen on brain function by the development of premenstrual syndrome (PMS): and behaviour is beyond the scope of this article and has a constellation of adverse psychological symptoms that recently been reviewed elsewhere (6). include worsening of mood and episodes of aggressive behaviour, often triggered by relatively minor stressors Responsiveness to stress varies during the related to everyday life (3). PMS and its more severe coun- estrous cycle in rodents terpart premenstrual dysphoric disorder (PMDD) interferes significantly with the woman’s quality of life, impairing work, Female rodents, like women, show estrous cycle-linked

Correspondence: T.A. Lovick, School of Clinical and Experimental Medicine, University of Birmingham, Birmingham B15 2TT, UK. E-mail: [email protected]

Presented at the III Fórum em Neurobiologia do Estresse, Araraquara, SP, Brazil, September 8-10, 2011.

Received November 26, 2011. Accepted March 13, 2012. Available online March 30, 2012. Published April 9, 2012.

Braz J Med Biol Res 45(4) 2012 www.bjournal.com.br Stress and the female brain 315

changes in responsiveness to stressful situations. In gen- mentally by withdrawal from an exogenous progesterone eral, higher levels of anxiety and responsiveness to mild dosing regimen (17,22,23,27,28) or occurring naturally as anxiogenic stress have been reported in the non-receptive a response to natural fluctuations during the estrous cycle phase of the cycle (diestrus) compared to the receptive (10-12,18), falling progesterone is anxiogenic in a variety phase (proestrus and estrus) in both rats and mice (7-13, of behavioural tests. but see Ref. 14 for an opposite view). Unlike women in The periaqueductal gray matter (PAG) is a midbrain whom the late luteal (premenstrual) phase is character- region recognised as key to mediating behavioural and ised by falling levels of both estrogen and progesterone, autonomic components of responses to stressful chal- the latter part of diestrus in rats, when responsiveness to lenge. As such, it has the potential to provide a useful brain stress is increased, is associated with a rapid fall only in locus for investigating the mechanisms underlying female plasma concentration of progesterone, whilst estrogen hormone-induced links between transcriptional changes at secretion remains at a stable low level (15). Since proges- the receptor level and changes in neuronal excitability and terone passes the blood brain barrier readily, fluctuations behaviour. As with other brain regions, the vast majority in peripheral secretion will be reflected in the brain. Falling of studies on this region have been carried out in males. concentration of progesterone in the brain could therefore However, as the results from more studies on females be- be the key factor that triggers increased responsiveness gin to emerge, it is becoming clear not only that the PAG to acute stress during late diestrus in rats and, by analogy, is involved in processing stress-evoked behaviour in the during the late luteal phase in women. In support of this female sex, as in males, but also that the excitability of its idea, behavioural changes similar to those seen in late circuitry fluctuates during the estrous cycle. In females in diestrus can be induced in rats by withdrawal from long- late diestrus, the fall in brain concentration of progester- term dosing with exogenous progesterone (16-19). The one, and hence ALLO, that occurs during this stage, trig- behaviour of spontaneously cycling rats in late diestrus gers up-regulation of extrasynaptic α4βδ GABAA receptor therefore appears to offer a useful model in which to study expression in the PAG, which is confined principally to the the role of progesterone in the development of premenstrual GABAergic neuronal subpopulation (Figure 1). This tran- syndrome in women. scriptional change correlates with an increase in functional excitability of the intrinsic PAG circuitry that translates in Falling progesterone leads to changes in the conscious animal to raised anxiety levels and increased neuronal excitability behavioural responsiveness to acute psychogenic stress (Figure 2A). At the cellular level electrophysiological studies There are several indications that rapid fluctuations in in spontaneously cycling rats in vivo have demonstrated endogenous steroid concentrations result in altered GABAA that GABAergic tone on output neurons within the PAG is receptor kinetics, which impacts on neuronal excitability. In reduced during the late diestrous phase (29), which renders rats, withdrawal from progesterone, either during the late the circuitry intrinsically more excitable. Direct evidence for diestrus phase in cycling animals or following an exog- an increase in functional excitability of the PAG circuitry enous progesterone withdrawal dosing regimen, has been during late diestrus has been provided by the recent report shown to evoke up-regulation of α4, β1 and δ subunits of of a lowered threshold for escape behaviour evoked by the GABAA receptor in several brain structures (7,19-23). direct electrical stimulation in the PAG in late diestrus (30). The effect was due not to progesterone itself but to the Autonomic responsiveness to a pentagastrin, a panicogenic fall in concentration of its neuroactive metabolite allopreg- agent known to activate PAG circuitry following systemic nanolone (ALLO), which triggers enhanced transcription of administration, was also increased during the late diestrous the genes encoding α4, β1 and δ subunits of the GABAA phase (31). At this time, functional activation of PAG circuitry receptor (16,17,23). Alpha4, β1 and δ subunits are able to evoked by acute exposure to a mild anxiogenic stress is also assemble into functional receptors. Recombinant α4β1δ changed, as reflected by decreased regional expression of GABAA receptors show electrophysiological properties the immediate early gene c-fos (32 and Figure 2B). At the characteristic of other extrasynaptic receptors containing present time the phenotype of the Fos-positive population δ subunits. Typically, these act tonically to modify the level in the PAG has not been determined. However, the find- of ongoing GABAergic inhibitory activity that regulates the ings are consistent with a stress-induced decreased c-fos excitability of neuronal ensembles (22,24-26). expression in the GABAergic population, and at the same time with an increase in c-fos expression in PAG output Functional consequences of progesterone neurons, which are likely to be less numerous than the withdrawal GABAergic population. Interestingly, in hippocampal tissue withdrawal from There is now compelling evidence that a reduction progesterone evoked an increased α4 receptor expression in brain concentration of progesterone enhances stress- in the hippocampus and a reduction in total GABA-stimulated induced anxiety-linked behaviours. Whether induced experi- current measured in dissociated pyramidal neurons (17,33). www.bjournal.com.br Braz J Med Biol Res 45(4) 2012 316 T.A. Lovick

Figure 1. Schematic drawing showing the functional consequences of increased α4β1δ GABAA receptor expression in the periaqueductal gray matter (PAG). A, When expression of α4β1δ receptors is low, spontaneous activity in GABAergic interneurones in the PAG limits the excitability of the output neurones. B, Increased expression of α4β1δ receptors when progesterone levels fall in late diestrous leads to an increase in tonic current carried by GABAergic cells, which limits their ongoing activity. The output neurones therefore become intrinsically more excitable, and the threshold for activation by stressful stimuli is lowered. Repro- duced from Ref. 41 with permission.

Figure 2. A, Change in latency of the tail flick reflex (s) following exposure to 5-min anxiogenic vibration stress in rats at different stages of the estrous cycle. Data (mean ± SEM) are reported as change in tail flick latency (TFL) from baseline in 6-min post- vibration stress. B, Density of Fos-positive neurons in the periaqueductal gray matter (PAG) of rats exposed to anxiogenic stress and tail flick testing at different stages of the estrous cycle. P = proestrus; E = estrus; ED = early diestrus; LD = late diestrus. Figure drawn from data in Ref. 32 and reproduced with permission. *P < 0.05, **P < 0.01 (post hoc Bonferroni test) stressed group compared to the non-stressed group after significant (P < 0.05) one-way ANOVA. §§P < 0.01 LD stressed group compared to stressed rats at all other stages of the estrous cycle (post hoc Bonferroni test).

Braz J Med Biol Res 45(4) 2012 www.bjournal.com.br Stress and the female brain 317

No effects were observed in cortex or cerebellum. Thus, gesterone levels might therefore be effective in preventing there may be regional differences with respect to the effects the withdrawal effect. triggered by withdrawal from progesterone. Moreover, at the cellular level the functional outcome of receptor upregulation Neurosteroid replacement in late diestrus appears to be dependent on the cell type that expresses prevents the development of stress-induced α4 subunit-containing GABAA receptors, e.g., interneurons hyperalgesia or principal neurons. We hypothesized that short-term neurosteroid replace- The kinetics of progesterone withdrawal ment, timed to blunt the rate of the natural fall in ALLO that determines its anxiogenic effect occurs during the late diestrous phase in spontaneously cycling rats, should prevent the increased responsiveness to It is becoming clear that withdrawal from progester- stress that normally occurs at this time (Figure 3). In males one during the late diestrous phase induces a rebound the selective serotonin reuptake inhibitor (SSRI) fluoxetine hyperexcitable state manifested by significant changes has been shown to produce rapid onset steroidogenic ef- in responsiveness to anxiogenic stress. Interestingly, the fects following acute administration, by raising plasma and critical event for inducing this effect may be related to the brain concentrations of ALLO (36-38). Importantly, these kinetics of the decline in steroid levels rather than the effects could be induced at doses below the threshold for absolute magnitude of the fall. In a study on female rats effects on 5-hydroxytryptamine (5-HT) systems (37). We chronically treated with estrogen and progesterone, a have shown that short-term administration of a low dose rapid withdrawal of these hormones precipitated increased of fluoxetine (1.75 mg/kg, ip) was also steroidogenic in startle and anxiety-like behaviour, which was not seen if the females, producing a doubling of whole brain ALLO con- hormone levels were allowed to decline gradually (34,35). centration (39). Since similar increases in anxiety behaviour can be pre- Compared to other cycle stages, rats in late diestrus cipitated by abrupt withdrawal from progesterone alone normally show an exaggerated response to anxiogenic (16-18), the trigger for anxiogenesis in the former study is vibration stress, which is manifested as hyperalgesia (18). likely to be the rapid decline in concentration in the brain of Administering low-dose fluoxetine to female rats on the progesterone, and hence its neuroactive metabolite ALLO. evening of early diestrus, with a top up next morning when Measures to produce a more gradually tapered fall in pro- they had reached late diestrus, completely prevented the

Figure 3. Schematic presentation of the rationale behind short-term neurosteroid replacement to prevent development of progesterone withdrawal-evoked anxiogenesis during late diestrus. A, At late diestrus, the fall in brain concentration of progesterone and its neuroactive metabolite allopregnanolone (ALLO) triggers a withdrawal syndrome manifested as an increased responsiveness to stress. B, Short-term dosing with a steroidogenic agent in the evening of early diestrus should raise plasma and hence brain allopregnanolone transiently and so blunt the rapid fall in allopregnanolone that normally occurs during late diestrus. The trigger for development of the withdrawal syndrome should be removed. www.bjournal.com.br Braz J Med Biol Res 45(4) 2012 318 T.A. Lovick

development of stress-induced hyperalgesia (Figure 4A). Similarly, the functional deactivation within the PAG that is normally induced by exposure to the vibration stress in late diestrus was prevented as well (Figure 4B). Importantly, the behavioural effects of fluoxetine could be reproduced following dosing with ganaxolone (data not shown), a syn- thetic analogue of ALLO not reported to have effects on 5-HT systems (40). Thus, the effect of fluoxetine is likely to be due to its steroidogenic action. The dramatic effects of short-term neurosteroid replacement in preventing the appearance of estrous cycle-linked anxiogenesis in rats may have relevance to the quest for more effective treatments of menstrual cycle-linked dysphoria in women. In conclusion, the data summarized here show how cyclical changes in production of progesterone during the estrous cycle can have a powerful effect on functioning of the female brain. In rodents, this occurs over a remark- ably short time frame. The withdrawal effects evoked by the natural cyclical variation in secretion of progesterone during the estrous cycle occur over 12- to 24-h periods, influencing anxiety levels and responsiveness to anxiogenic stress. To date the overwhelming majority of the studies on brain function and behaviour in animals has been confined to males, a common argument for the single sex approach being that data obtained from females are more variable and may be confounded by hormonal influences. Increasingly, however, it is being recognized that sex differences do exist, both in terms of behaviour and responsiveness to certain drugs. It is essential that the effects of possible influences of the ovarian cycle on brain function and behaviour are Figure 4. A, Short-term dosing with fluoxetine (1.75 mg/kg) on incorporated into experimental designs in future studies the evening of late diestrus, with a top up on the morning of late using females. In a world in which 50% of the population diestrus, prevents the development of stress-induced hyper- is female, such differences are important and should no algesia during late diestrus (decrease in tail flick latency (TFL) compared to naive non-stressed rats). White bars = non-stressed longer be overlooked. rats; light gray bars = stressed vehicle-treated rats; dark gray bars = stressed fluoxetine-treated rats. B, Fluoxetine administration also prevented the reduction in stress-induced functional deactivation in the periaqueductal gray matter (PAG) (decrease in density of Fos-positive neurons in the PAG compared to naive non-stressed rats; Devall AJ and Lovick TA, unpublished results). *P < 0.05 compared to vehicle-treated rats in late diestrus (one- way ANOVA followed by the Bonferroni test).

References

1. Legato MJ. The skewed sex distribution in affective disor- 2003; 17: 325-342. ders - a diagnostic, social, or biological problem? Prog Brain 5. Hammarback S, Ekholm UB, Backstrom T. Spontaneous Res 2010; 186: 159-166. anovulation causing disappearance of cyclical symptoms in 2. Paul SM, Purdy RH. Neuroactive steroids. FASEB J 1992; women with the premenstrual syndrome. Acta Endocrinol 6: 2311-2322. 1991; 125: 132-137. 3. O’Brien S, Rapkin A, Dennerstein L, Nevatte T. Diagnosis 6. Handa RJ, Ogawa S, Wang JM, Herbison AE. Roles for and management of premenstrual disorders. BMJ 2011; 342: oestrogen receptor beta in adult brain function. J Neuroen- d2994. docrinol 2012; 24: 160-173. 4. Backstrom T, Andreen L, Birzniece V, Bjorn I, Johansson 7. Gangitano D, Salas R, Teng Y, Perez E, De Biasi M. Pro- IM, Nordenstam-Haghjo M, et al. The role of hormones and gesterone modulation of alpha5 nAChR subunits influences hormonal treatments in premenstrual syndrome. CNS Drugs anxiety-related behavior during estrus cycle. Genes Brain

Braz J Med Biol Res 45(4) 2012 www.bjournal.com.br Stress and the female brain 319

Behav 2009; 8: 398-406. 397-405. 8. Gouveia A Jr, dos Santos UD, Felisbino FE, de Afonseca TL, 23. Gulinello M, Orman R, Smith SS. Sex differences in anxiety, Antunes G, Morato S. Influence of the estrous cycle on the sensorimotor gating and expression of the alpha4 subunit behavior of rats in the elevated T-maze. Behav Processes of the GABAA receptor in the amygdala after progesterone 2004; 67: 167-171. withdrawal. Eur J Neurosci 2003; 17: 641-648. 9. Ho HP, Olsson M, Westberg L, Melke J, Eriksson E. The 24. Mody I. Aspects of the homeostatic plasticity of GABAA serotonin reuptake inhibitor fluoxetine reduces sex steroid- receptor-mediated inhibition. J Physiol 2005; 562: 37-46. related aggression in female rats: an animal model of pre- 25. Smith SS, Gong QH. Neurosteroid administration and with- menstrual irritability? Neuropsychopharmacology 2001; 24: drawal alter GABAA receptor kinetics in CA1 hippocampus 502-510. of female rats. J Physiol 2005; 564: 421-436. 10. Koonce CJ, Walf AA, Frye CA. Type 1 5alpha-reductase may 26. Wei W, Zhang N, Peng Z, Houser CR, Mody I. Perisynaptic be required for estrous cycle changes in affective behaviors localization of delta subunit-containing GABA(A) receptors of female mice. Behav Brain Res 2012; 226: 376-380. and their activation by GABA spillover in the mouse dentate 11. Marcondes FK, Miguel KJ, Melo LL, Spadari-Bratfisch RC. gyrus. J Neurosci 2003; 23: 10650-10661. Estrous cycle influences the response of female rats in the 27. Gallo MA, Smith SS. Progesterone withdrawal decreases elevated plus-maze test. Physiol Behav 2001; 74: 435- latency to and increases duration of electrified prod burial: 440. a possible rat model of PMS anxiety. Pharmacol Biochem 12. Mora S, Dussaubat N, Diaz-Veliz G. Effects of the estrous Behav 1993; 46: 897-904. cycle and ovarian hormones on behavioral indices of anxiety 28. Lofgren M, Johansson IM, Meyerson B, Lundgren P, Back- in female rats. Psychoneuroendocrinology 1996; 21: 609- strom T. Progesterone withdrawal effects in the open field 620. test can be predicted by elevated plus maze performance. 13. Reddy DS, Kulkarni SK. Sex and estrous cycle-dependent Horm Behav 2006; 50: 208-215. changes in neurosteroid and benzodiazepine effects on 29. Brack KE, Lovick TA. Neuronal excitability in the periaque- food consumption and plus-maze learning behaviors in rats. ductal grey matter during the estrous cycle in female Wistar Pharmacol Biochem Behav 1999; 62: 53-60. rats. Neuroscience 2007; 144: 325-335. 14. Hiroi R, Neumaier JF. Differential effects of ovarian steroids 30. Santos JM, Lovick TA, Brandão ML. Effects of acute treat- on anxiety versus fear as measured by open field test and ment with fluoxetine on the neural substrates of fear of the fear-potentiated startle. Behav Brain Res 2006; 166: 93- periaqueductal gray matter in female rats. Neuroscience 100. 2011; Abstract 281.02. 15. Butcher RL, Collins WE, Fugo NW. Plasma concentration 31. Brack KE, Jeffery SM, Lovick TA. Cardiovascular and respi- of LH, FSH, prolactin, progesterone and estradiol-17beta ratory responses to a panicogenic agent in anaesthetised throughout the 4-day estrous cycle of the rat. Endocrinology female Wistar rats at different stages of the oestrous cycle. 1974; 94: 1704-1708. Eur J Neurosci 2006; 23: 3309-3318. 16. Smith SS, Gong QH, Hsu FC, Markowitz RS, ffrench-Mullen 32. Devall AJ, Santos JM, Lovick TA. Estrous cycle stage influ- JM, Li X. GABA(A) receptor alpha4 subunit suppression ences on neuronal responsiveness to repeated anxiogenic prevents withdrawal properties of an endogenous steroid. stress in female rats. Behav Brain Res 2011; 225: 334- Nature 1998; 392: 926-930. 340. 17. Smith SS, Gong QH, Li X, Moran MH, Bitran D, Frye CA, 33. Bitran D, Smith SS. Termination of pseudopregnancy in the et al. Withdrawal from 3alpha-OH-5alpha-pregnan-20-One rat produces an anxiogenic-like response that is associated using a pseudopregnancy model alters the kinetics of hip- with an increase in benzodiazepine receptor binding density pocampal GABAA-gated current and increases the GABAA and a decrease in GABA-stimulated chloride influx in the receptor alpha4 subunit in association with increased anxi- hippocampus. Brain Res Bull 2005; 64: 511-518. ety. J Neurosci 1998; 18: 5275-5284. 34. Doornbos B, Fokkema DS, Molhoek M, Tanke MA, Postema 18. Devall AJ, Liu ZW, Lovick TA. Hyperalgesia in the setting of F, Korf J. Abrupt rather than gradual hormonal changes in- anxiety: sex differences and effects of the oestrous cycle in duce postpartum blues-like behavior in rats. Life Sci 2009; Wistar rats. Psychoneuroendocrinology 2009; 34: 587-596. 84: 69-74. 19. Gangisetty O, Reddy DS. Neurosteroid withdrawal regulates 35. Saavedra M, Contreras CM, Azamar-Arizmendi G, Hernan- GABA-A receptor alpha4-subunit expression and seizure dez-Lozano M. Differential progesterone effects on defen- susceptibility by activation of progesterone receptor-inde- sive burying and forced swimming tests depending upon pendent early growth response factor-3 pathway. Neurosci- a gradual decrease or an abrupt suppression schedules. ence 2010; 170: 865-880. Pharmacol Biochem Behav 2006; 83: 130-135. 20. Griffiths J, Lovick T. Withdrawal from progesterone increases 36. Uzunov DP, Cooper TB, Costa E, Guidotti A. Fluoxetine- expression of alpha4, beta1, and delta GABA(A) receptor elicited changes in brain neurosteroid content measured by subunits in neurons in the periaqueductal gray matter in negative ion mass fragmentography. Proc Natl Acad Sci U female Wistar rats. J Comp Neurol 2005; 486: 89-97. S A 1996; 93: 12599-12604. 21. Griffiths JL, Lovick TA. GABAergic neurones in the rat pe- 37. Pinna G, Costa E, Guidotti A. SSRIs act as selective brain riaqueductal grey matter express alpha4, beta1 and delta steroidogenic stimulants (SBSSs) at low doses that are GABAA receptor subunits: plasticity of expression during inactive on 5-HT reuptake. Curr Opin Pharmacol 2009; 9: the estrous cycle. Neuroscience 2005; 136: 457-466. 24-30. 22. Lovick TA, Griffiths JL, Dunn SM, Martin IL. Changes in 38. Serra M, Pisu MG, Muggironi M, Parodo V, Papi G, Sari R, et GABA(A) receptor subunit expression in the midbrain during al. Opposite effects of short- versus long-term administration the oestrous cycle in Wistar rats. Neuroscience 2005; 131: of fluoxetine on the concentrations of neuroactive steroids www.bjournal.com.br Braz J Med Biol Res 45(4) 2012 320 T.A. Lovick

in rat plasma and brain. Psychopharmacology 2001; 158: 40. Nohria V, Giller E. Ganaxolone. Neurotherapeutics 2007; 4: 48-54. 102-105. 39. Devall A, Fry JP, Honour JW, Lovick TA. Short term, low 41. Lovick TA. Plasticity of GABAA receptor subunit expres- dose fluoxetine treatment increases brain allopregnanolone sion during the oestrous cycle of the rat: implications for concentrations in female rats and abolishes estrous cycle- premenstrual syndrome in women. Exp Physiol 2006; 91: related stress-induced hyperalgesia? Proc Physiol Soc 655-660. 2011; 22; PC11.

Braz J Med Biol Res 45(4) 2012 www.bjournal.com.br